In a US defense program, there are two production milestones. The first is low rate initial production, which the F35 passed some time ago. That milestone can be passed even if there is still operational testing that is not completed, but it only allows a limited number of units to be produced, and, as the name implies, the rate of production is slower to allow for changes based on results from testing. The second milestone is full rate production, which can't be passed until all operational testing is completed. Once that is passed the full number of units can be produced.

I think it's a contract checkpoint that signals a major $$$ release to the vendor.

Basically, "will we make the several thousand we originally planned to make?"

For example, the B-2 was planned with over a hundred aircraft, but we wound up making only about 20.

600 have already been delivered. Covid caused a disruption in final certification, that's it. The F35 isn't being cancelled, although I predict most comments here will devolve into a debate of the plane's merits. The US and many allies are committed, the US will eventually own thousands.